Lucky Imaging

NEWS: AOLI First Light on the WHT (4.2m)

It is just a year since the STFC awarded a research grant to provide part of the support for developing AOLI. AOLI is an Instrument for near-diffraction limited imaging in the visible on ground-based telescopes. The technique has already been demonstrated on the WHT by our team with FastCam+NAOMI, and on the Palomar 5m, giving 35 mas resolution in I-band (>3 times Hubble) . Our goal is to provide this performance with close to all-sky coverage, using faint reference stars (I~17-18) and reaching science targets as faint as Hubble. AOLI achieves these faint reference star limits by using a novel low-order curvature wavefront sensor with photon-counting EMCCD detectors. We have just completed our first commissioning run on the WHT which was very successful in that virtually all aspects of the instrument performed as intended. However, exceptionally poor observing conditions (seeing often worse than 3-4 arcseconds with high humidity and little sky time) greatly limited the amount of data taken. Nevertheless valuable experience was gained and the prospects are excellent for using AOLI for science measurements in a later semester. A full report may be found here.

About Lucky Imaging

Lucky Imaging is a remarkably effective technique for delivering near-diffraction-limited imaging on ground-based telescopes. The basic principle is that the atmospheric turbulence that normally limits the resolution of ground-based observations is a statistical process. If images are taken fast enough to freeze the motion caused by the turbulence we find that a significant number of frames are very sharp indeed where the statistical fluctuations are minimal. By combining these sharp images we can produce a much better one than is normally possible from the ground. We have routinely taken Hubble resolution images (0.15 arcsec resolution) on the Hubble sized telescope is (~2.5 m). More recently we have used the same techniques behind a low order adaptive optics system in order to give even higher resolution on telescopes that are too big to have a significant chance of conventional lucky imaging without adaptive optics assistance.

Lucky imaging is not a new idea. The name "Lucky Imaging" came from Fried (1978) though the first calculations of the Lucky Imaging probabilities were first carried out by Hufnagel in 1966 (see reference pages (click here) for copies of the Hufnagel papers that are otherwise difficult to find) and these principles have been used really quite extensively by the amateur astronomy community who have been able to take very high quality images of bright objects such as Mars and the other planets. There is more information about Amateur Lucky Imaging here..

Recent Publications

A recent paper by Craig Mackay looks at ways in which the overall efficiency of Lucky Imaging might be improved. One of its perceived disadvantages is that it does require that are relatively large percentage of the images recorded are discarded. By changing the lucky image selection criteria to do some of the selection in the Fourier plane much higher selection percentages may be used. The paper will be published soon in the Monthly Notices of the Royal Astronomical Society and is entitled "High-Efficiency Lucky Imaging". The abstract of the paper is as follows:

"Lucky Imaging is now an established observing procedure that delivers near diffraction-limited images in the visible on ground-based telescopes up to ~2.5 m in diameter. Combined with low order adaptive optics it can deliver resolution several times better than that of the Hubble Space Telescope. Many images are taken at high speed as atmospheric turbulent effects appear static on these short timescales. The sharpest images are selected, shifted and added to give a much higher resolution than is normally possible in ground-based long exposure time observations. The method is relatively inefficient as a significant fraction of the frames are discarded because of their relatively poor quality. This paper shows that a new Lucky Imaging processing method involving selection in Fourier space can substantially improve the selection percentages. The results show that high resolution images with a large isoplanatic patch size may be obtained routinely both with conventional Lucky Imaging and with the new Lucky Fourier method. Other methods of improving the sensitivity of the method to faint reference stars are also described."

A second paper has recently been published by Peter Aisher et al., that looks at various strategies that may be used in fitting wavefronts when multiple near-pupil plane images have been recorded. The abstract is as follows:

"Increasing interest in astronomical applications of non-linear curvature wavefront sensors for turbulence detection and correction makes it important to understand how best to handle the data they produce, particularly at low light levels. Algorithms for wavefront phase retrieval from a four-plane CWFS are developed and compared, with a view to their use for loworder phase compensation in instruments combining adaptive optics and Lucky Imaging. The convergence speed and quality of iterative algorithms is compared to their step-size, and techniques for phase retrieval at low photon counts are explored. Computer simulations show that at low light levels, preprocessing by convolution of the measured signal with a Gaussian function can reduce by an order of magnitude the photon flux required for accurate phase retrieval of low-order errors. This facilitates wavefront correction on large telescopes with very faint reference stars."

AOLI Is a major collaboration between the University of Cambridge, Institute of Astronomy, the IAC in La Laguna, Tenerife, the ING in La Palma and the Universities of Cartegena and Cologne. The acronym AOLI stands for “Adaptive Optics Lucky Imager”. This project aims at building a camera able to deliver diffraction limited images in the visible range. This instrument is first designed for the 4.2-m William Herschel Telescope, in the island of La Palma (Canary Islands), but later intended for use on the 10.4-m Gran Telescopio de Canarias (GTC).

Obtaining optical diffraction limited images is almost impossible to achieve from the ground because of the lack of efficient adaptive-optics systems for wavelength below 1.2-1.6 microns. The atmospheric turbulence rapidly degrades the wavefronts entering the telescope, which results in seeing-limited images with no spatial information below 0.8-1” resolution. Until recently, optical diffraction-limited images were only delivered by the Hubble Space Telescope

By combining Lucky Imaging techniques with low order adaptive optics it is possible to obtain diffraction limited images in the visible on ground-based telescopes. AOLI is intended to give a resolution of approximately 3 times that of the Hubble Space Telescope from the ground on the WHT 4.2 m telescope and approximately 8 times that Of the Hubble Space Telescope on the 10.5 m GTC telescope

Observing and Results

A recent observing trip (July 2007) to the Palomar 200 inch telescope has been extremely successful. The images we obtained are the highest resolution direct images, about 35 milliarcsec FWHM, ever obtained either from the ground or from space in the visible at about twice the resolution of the Hubble Space Telescope.

M13

Images shown below are of the core of globular cluster M 13. The blinking image shows what the telescope delivers on its own, followed by what it delivers with the adaptive optic system and LuckyCam.

The images below show a direct comparison between the conventional image taken under conditions of good seeing (0.65 arcsec), the Hubble image from the ACS (centre) and our Lucky/AO image (right). The Hubble picture goes fainter because the exposure is longer and the wavelength shorter (where CCDs have a much higher sensitivity). The ACS image has been "drizzled" to improve its appearance. The Lucky image is as taken. The markedly better resolution of the Lucky image is clear. This is exactly what is predicted purely because the Palomar 5.1 m telescope is twice the size of the 2.5m Hubble.

Cat's Eye Nebula (NGC 6543)

A planetary nebula is formed when the central star evolves from a red giant to its final white dwarf phase. A relatively short time in the life of the star, possibly 10,000 years in total, gas is ejected from the surface of the dying star. We can look at the expansion velocity of these filaments and sure that the age of the bright inner shells is probably only about 1000 years. The nebula is about 3000 light years from Earth.

The image below shows the Cat’s Eye Nebula (NGC 6543) as imaged conventionally by the Palomar 200 inch telescope. The green light is oxygen emission, the red is hydrogen emission, and the blue is near-infrared radiation, again followed by the Cat’s Eye Nebula (NGC7543) as imaged with the Lucky Camera behind an adaptive optics system on the Palomar 200 inch telescope. The resolution in the Lucky image is lower than Hubble as the image covers four times the area of the M13 images above, but it is still a good demonstration of what can be done from the ground. These images are all slightly lower resolution than those of the globular cluster but nevertheless show the considerable improvement over conventional ground-based imaging that the AO system produces with LuckyCam.